ADCs and DACs — Data Converters

Overview

Selecting and interfacing ADCs and DACs requires attention to sampling theory, input conditioning, reference selection, and layout to preserve accuracy. This chapter focuses on practical converter selection and measurement techniques.

Prerequisites

• Familiarity with sampling concepts and signal conditioning basics

Learning objectives

• Choose ADC/DAC types suitable for an application (SAR, delta-sigma, pipeline)
• Design anti‑alias filters and input conditioning to match converter requirements
• Measure converter performance (SNR, THD, ENOB) using practical setups

Tools & materials

• Function generator, precision reference, oscilloscope, data acquisition tool or microcontroller, anti‑alias filter components

Hands-On Mini Task

1. Interface a SAR ADC to a simple anti‑alias filter and measure a sinewave input at different sample rates. Compute SNR and compare to datasheet expectations.
2. For a DAC, generate a waveform and measure linearity and noise at the output with proper buffering.

Expected result: measured SNR/THD in reasonable agreement with datasheet, and clear effects of input conditioning on performance.

Sampling theory and anti-aliasing

• Nyquist: sample rate must exceed twice the highest signal frequency; practical systems include an anti-alias filter to remove high-frequency content.
• Choose filter order and cutoff to balance aliasing suppression with passband flatness.

References and input conditioning

• Use low-noise references and buffer inputs where necessary; input impedance must match source and converter requirements.
• Consider driver amplifiers and proper filtering to match ADC input sampling capacitors and hold behaviour.

Measuring converter performance

• SNR: ratio of signal power to noise power in the bandwidth; related to ENOB (effective number of bits).
• THD and SINAD: measure distortion products and combined noise/distortion metrics using FFT analysis tools.

Worked example — measuring SNR

1. Generate a pure sine into the ADC with amplitude matching the input range. Sample and compute FFT to extract SNR and THD.
2. Compare measured ENOB = (SNR - 1.76) / 6.02 to datasheet values.

Troubleshooting

• If measured SNR is lower than expected, check reference stability, input grounding, and routing near digital sections.
• Aliasing artefacts often show up as spurs in the spectrum — confirm anti-alias filter performance and sample rate selection.

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